The present invention relates generally to drawing compasses and specifically to an improved writing implement or pencil clamp for use in conjunction with a drawing compass. While the present invention is intended to be used with drawing compasses using any writing implement, the use of a pencil is described herein. It should be appreciated that wherever the term "pencil" is used, any writing implement can be substituted unless the context indicates otherwise.
The use of a compass to draw a circle or a portion of an arc is well known in the art and a large variety of differing compasses have been produced. The present invention is concerned specifically with that type of compass which employs the use of a writing implement, preferably a pencil, to describe the arc or circle produced. Basically, a drawing compass comprises two legs which are connected together at their top ends by a pivot rivet. A pintle or handle is secured to the pivot rivet, which handle facilitates the spinning of that leg of the compass holding the pencil about the second leg, the second leg being provided with a sharp point and placed at the center of the circle or arc to be drawn.
The prior art pencil clamps extend from one leg of the compass and comprise a flexible cylindrical band having a top and bottom opening, thus defining a central channel. The cylindrical band is longitudinally split to define a side opening and two edges; each edge being provided with an outwardly extending ear. A pencil passes through the central channel and is held in position by a clamping mechanism. This clamping mechanism comprises a clamping lever having an eccentric shaped slot through which the ears of the cylindrical band pass. By turning the clamping lever the continuous wall of the slot serves as a camming wall for the ears thereby drawing the edges of the cylindrical band closer together, narrowing the width of the side opening, and thus clamping the cylindrical band about the pencil.
The drawing compass is a relatively inexpensive item and is generally designed for mass production manufacture and low selling price. A persistent problem which had existed in the manufacture of drawing compasses has been the difficulty in producing a mechanism which will easily, accurately and securely clamp the pencil within the compass and at the same time, be relatively inexpensive to manufacture.
Included within the disadvantages of the prior art clamps for drawing compasses has been their general failure to solve the problems connected with the requirement for easy removal of the pencil for sharpening, the accurate initial positioning of the pencil point upon insertion of the pencil into the clamp, the readjustment of the pencil position relative to the pointed stationary leg of the compass as the pencil point becomes worn, the ability of the compass pencil clamp to fixedly secure pencils of varying diameters and the physical damage to the pencil due to the clamping action.
While some of these problems were solved by the compass constructed in accordance with U.S. Pat. No. 3,020,641, this compass construction suffers from the disadvantage that it required that the clamping lever be provided with a specially designed and machined slot. The slot provides camming walls for the ears of the cylindrical band thereby facilitating the opening, closing and locking of the clamp about the pencil.
Generally, it is an object of the present invention to provide a pencil clamp for use in a drawing compass which eliminates the disadvantages of the prior art pencil clamps as described above. Specifically, it is an object of the present invention to provide a pencil clamp for a drawing compass which will fixedly, accurately and firmly secure a pencil, including pencils having relative small diameters, to one leg of the compass and which will, in addition, allow for the selective easy positioning and movement of the pencil. It is thus an object of the present invention to provide a pencil clamp which holds normal-sized pencils securely in place and, in addition, is adapted to hold and secure pencils having relatively smaller diameters.
Another object of the present invention is to eliminate the necessity for specially machining the slot of the clamping lever by providing a clamping lever having a slot which is far simpler, cheaper and easier to manufacture.
Still further, it is an object of the invention to provide a slot for a clamping lever which provides a camming wall which enables the cylindrical band to tightly clamp the writing implement, to an even tighter degree than capable of being performed by the prior art clamps.